The design of radio frequency (xe2x80x9cRFxe2x80x9d) power amplifiers is typically accomplished by taking into account the expected operating environment of the amplifier and the desired output characteristics of the amplifier, such as the output load line impedance. There are usually interrelated yet contradictory requirements that must be taken into account in setting the output load line impedance and trade-offs must be made that affect the performance of the amplifier. Some of the factors to be considered in setting the output load line impedance are output peak and average signal requirements, distortion limits, efficiency requirements, and the available power supply voltage. The trade-off problem is made more acute when the amplifier is to be designed for high peak to average signal conditions, such as those found in Trellis Coded 8-Level Vestigial Side Band (xe2x80x9c8VSBxe2x80x9d) and/or Coded Orthogonal Frequency Division Multiplexing (xe2x80x9cCOFDMxe2x80x9d) television transmitters.
For instance, one example of the contradictory requirements and trade-offs that need to be made involves the distortion limit of an amplifier. Typically, a designer strives to keep distortion to a minimum and amplifier efficiency to a maximum. The distortion limit is set by the maximum linear amplifier output power of the amplifier. The maximum linear output is a function of the amplifier output load line impedance and the voltage supplied to the amplifier by a power supply, e.g., the drain source power supply. A lower output load line impedance and a higher amplifier supply voltage typically result in a higher maximum linear output (i.e., a higher distortion limit), all other factors being relatively constant. A higher distortion limit allows for more amplifier head room before the negative effects of distortion, such as non-linear amplification and a lack of spectral containment, need to be considered. However, the efficiency of the amplifier depends on the output signal level in relation to the maximum linear output such that amplifier efficiency increases as the output signal level increases up to and beyond the maximum linear output power of the amplifier. Therefore, maximum amplifier efficiency is attained in the distortion operating region.
As mentioned above, 8VSB and COFDM signals have high peak to average power ratios. An 8VSB signal has a peak to average power ratio in the range of 5 to 8 dB, typically around 6.5 dB. A COFDM signal has a peak to average power ratio in the range of 7 to 11 dB, typically around 9 dB. For either an 8VSB or COFDM signal, in order to have acceptable spectral containment of the output signal, signal peaks may only exceed the distortion limit, i.e., push the amplifier into its non-linear region, by a limited amount. As a result, the output signal average power is well below the amplifier""s maximum linear power capability therefore resulting in a low amplifier efficiency.
One drawback of a low amplifier efficiency is that the amplifier must dissipate the excess power as heat. Reliable amplifier design depends on, among other things, maintaining reasonable limits on transistor junction temperatures. Low amplifier efficiencies require cooling systems with a higher heat transfer capacity, and therefore higher cost, in order to prevent overheating and subsequent failure of the amplifier.
Where an amplifier is designed to operate over a range of frequencies (xe2x80x9cwide bandxe2x80x9d) the load line impedance will change due to the natural behavior of the amplifier""s output network as a function of the frequency of the input signal to the amplifier. Typical prior art wide band amplifiers are designed based on a minimum average output power capability and are operated at the same average output power capability at all frequencies.
However, a wide band amplifier designed for a minimum average output power capability over all frequencies in its range will have a higher capability, at most frequencies, to operate at an average output power that is greater than the minimum average output power capability. This is due to the fact that designing for a minimum average power capability means designing an output network whose load line is never higher (as input signal frequency varies) than the impedance dictated by the drain supply voltage for that power level. Since it is impossible to eliminate variations in the impedance transformation of the network, the impedance the network presents at other frequencies will always be lower than the design target impedance at the design minimum average output power capability. The reduced efficiency will define the worst case efficiency and hence the worst case excess power dissipation requirement.
The present invention takes advantage of the fact that it is possible to operate a wide band power amplifier at an applied supply voltage other than the operating voltage for a given design condition, such as the minimum target power condition described above. For example, if a wide band amplifier is operated at the same average output power at two frequencies and the load line impedance is higher for one of the two frequencies, the amplifier will operate at a higher efficiency where the load line impedance is higher. This is due to the fact that the amplifier makes more effective use of the applied voltage swing allowed by the applied power supply voltage, e.g., VCC where the impedance is higher. In the case where the load line impedance is lower for the same average output power, the amplifier is being supplied with more supply voltage than required by the amplifier resulting in an increase in excess power and a decrease in the efficiency of the amplifier. In order to decrease the excess power and thereby increase the efficiency of the amplifier, the amplifier supply voltage can be decreased.
Therefore, once the design of the amplifier and the electrical circuit to which the amplifier is connected is known, the behavior of the load line can be characterized as a function of one or more of the operating conditions of the amplifier and/or electrical circuit, specifically the frequency of the input signal to the amplifier. Other operating conditions that are related in a known manner to a change in the input signal frequency may be sensed. Therefore, the effects of the changing load line due to the changing operating conditions can be compensated for by adjusting the voltage supplied to the amplifier by the amplifier power supply as a function of one or more of the sensed operating conditions. Some of the operating conditions that may change, and therefore indicate a change in the input signal frequency, and can be sensed as affecting the load line impedance are the frequency of the input signal to the amplifier, the impedance of the electrical circuit to which the amplifier is connected, the efficiency of the amplifier, the modulation of the input signal to the amplifier, the ratio of peak power to average power of the input signal to the amplifier, and the ratio of peak voltage to average voltage of the input signal to the amplifier. It is to be understood that other operating conditions of the amplifier or the electrical circuit to which the amplifier is connected may also affect the load line impedance of the amplifier and are contemplated by the present invention.
The present invention overcomes the limitations of the prior art by enabling the voltage supplied to the amplifier by the amplifier power supply to be adjusted either dynamically as a function of one or more of the operating conditions described above or manually by an operator either in response to or independently of one or more of the operating conditions. For example, the operator may be informed as to the input signal frequency by sources not based on a measurement of the input signal frequency, such as when the operator is told by another (e.g., the system user) that the input signal frequency either is a certain value or will be changed to a certain value.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel amplifier system and method for dynamically adjusting the power supplied to an amplifier.
It is another object of the present invention to provide a novel amplifier system and method where a look up table is used in determining the amount of amplification to be applied by an amplifier to an input signal of any frequency within the frequency range of the amplifier and of any type of modulation.
It is yet another object of the present invention to provide a novel amplifier for amplifying an input signal as a function of one or more operating condition of the circuit to which the amplifier is connected by the use of a lookup table responsive to one or more of the operating conditions to determine the modification of the power supplied to the amplifier.
It is still another object of the present invention to provide a novel amplifier control circuit including a lookup table for effecting a modification in the voltage supplied to an amplifier in response to one or more operating conditions of the circuit to which the amplifier is connected, where the look up table is used in determining the relationship between the one or more operating conditions and a modification in the voltage supplied to the amplifier.
It is a further object of the present invention to provide a novel amplifier system and method for dynamically controlling the voltage applied to the amplifier through the use of a lookup table as a function of one or more of the following operating conditions: the frequency of the input signal to the amplifier, the impedance of the electrical circuit to which the amplifier is connected, the efficiency of the amplifier, the modulation of the input signal to the amplifier, the ratio of peak power to average power of the input signal to the amplifier, or the ratio of peak voltage to average voltage of the input signal to the amplifier.
It is yet a further object of the present invention to provide a novel amplifier system and method for dynamically controlling the voltage applied to the amplifier through the use of a lookup table as a function of the input signal to the amplifier where the input signal is either a COFDM signal or an 8VSB signal.
It is still a further object of the present invention to provide a novel amplifier system and method for dynamically controlling the amplification applied by an amplifier to an input signal through the use of a lookup table as a function of the input signal where the input signal is a COFDM signal with a peak to average power ratio in the range of 7 to 11 dB.
It is an additional object of the present invention to provide a novel amplifier system and method for dynamically controlling the amplification applied by an amplifier to an input signal through the use of a lookup table as a function of the input signal where the input signal is an 8VSB signal with a peak to average power ratio in the range of 5 to 8 dB.
It is yet an additional object of the present invention to provide a novel amplifier system and method for dynamically controlling the amplification applied by an amplifier to an input signal through the use of a lookup table as a function of the input signal where the input signal is a COFDM signal with a peak to average power ratio of approximately 9 dB.
It is still an additional object of the present invention to provide a novel amplifier system and method for dynamically controlling the amplification applied by an amplifier to an input signal through the use of a lookup table as a function of the input signal where the input signal is an 8VSB signal with a peak to average power ratio of approximately 6.5 dB.
It is a further additional object of the present invention to provide a novel television transmitter circuit including a power amplifier circuit capable of amplifying a variable frequency COFDM or 8VSB input signal where the amount of amplification applied to the input signal is dynamically controlled through the use of a lookup table as a function of the frequency of the input signal.
It is yet a further additional object of the present invention to provide a novel amplifier control circuit for dynamically adjusting the power supplied to an amplifier as a function of one or more of the operating conditions of the amplifier and/or the electrical circuit to which the amplifier is connected.
It is still a further additional object of the present invention to provide a novel amplifier system and method where the voltage applied to an amplifier can be either (a) dynamically controlled as a function of one or more operating condition of the amplifier and/or the electrical circuit to which the amplifier is connected, or (b) controlled by an operator via a user interface module in response to (i) one or more of the operating conditions, (ii) knowledge of the frequency of the input signal gained from other than a measurement of the input signal frequency, or (c) independently of the operating conditions.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.